Earless seal is the common name for any of the pinnipeds comprising the family Phocidae, characterized by the absence of a pinna (external part of ear, although there is a functional inner ear), a side-to-side swimming motion involving the hind-flippers and lower body, and hind-flippers that cannot be inverted forward under the body, making for slow and awkward motion on land. These characteristics distinguish phocids, also known as true seals or as crawling seals, from the eared seals (fur seals and sea lions) of the family Otariidae.

Earless seals live in the oceans of both hemispheres and are mostly confined to polar, sub-polar, and temperate climes, with the exception of the more tropical monk seals. Earless seals comprise about 90 percent of the species of pinnipeds and are the only seals in the extreme polar regions (Riedman 1990).

A number of earless seals have been commercially important for their hides or oil, such as the ringed seal, elephant seal, monk seals, and even pups of the harp seal. As a result of commercial exploitation, the elephant seal was nearly exterminated and monk sea populations were greatly depleted; the Caribbean monk seal may be extinct. Seals also have been an important food source, both for native populations, such as Eskimos, and as part of both marine and terrestrial food chains (sharks, orcas, bears).

Contents

Overview

Earless seals are one of the three main groups of mammals within the taxonomic groupPinnipedia. (Pinnipedia is usually considered a suborder of the order Carnivora, but is sometimes considered a separate order or as a superfamily.) Pinnipeds are aquatic (mostly marine) mammals that are characterized by having both front and hind limbs in the form of flippers. In addition to earless seals, other pinnipeds include walruses and eared seals (sea lions and fur seals).

Seals, which are any pinnipeds other than walruses, are placed into two groups: earless seals, comprising the taxonomic family Phocidae (phocids), and eared seals ((or "walking seals"), comprising the family Otariidae. Walruses generally are considered a separate family of pinnipeds, the Obobenidae, although sometimes they are included with the phocids (McKenna and Bell 1997).

Of the estimated 50 million pinnipeds in the world, roughly 90 percent are phocid seals, largely because of the 30 million or so crabeater seals (Lobodon sp.) in the Antarctic (Riedman 1990).

One way of differentiating between the two main groups of seals is by the presence of the pinna, a small furry earflap (external ears), found on the otarids and missing from phocids. Phocids are referred to as "earless seals" because their ears are not easily seen, while otarids are referred to as "eared seals." In addition to the presence of the pinna, there are other obvious differences between otarids and phocids. Otarids have hind-flippers that can be inverted under the body, aiding their movement on land, while the hind-flippers of phocids cannot be turned forward under the body, causing their movement on land to be slow and awkward (Riedman 1990). Otarids also swim using their long front flippers to move themselves through the water, while phocids swim by using their rear flippers and lower body in a side-to-side motion (Riedman 1990). There are also behavioral differences, including the breeding systems.

Only earless seals live in the Antarctic and Arctic; there are no otariids living and breeding in the extreme polar regions (Riedman 1990). On the other hand, only two species of phocids (the endangered Hawaiian and Mediterranean monk seals) are found in the tropics and these are small populations, while a number of fur seals and sea lions live in tropical and subtropical areas, with fur seals ranging widely into colder climates as well (Riedman 1990). The Antarctic fur seal, which breeds on islands between 45° S and 60° S, likely has a winter range that includes spending time close to the Antarctic ice.

Anatomy and behavior

Phocids are more highly specialized for aquatic life than otariids, although they still return to dry land or pack ice in order to breed and give birth. They have sleek, streamlined bodies. To further aid streamlining, their nipples can be retracted, their testicles are internal, and the penis lies in an internal sheath. A smooth layer of blubber lies underneath the skin, and phocids are able to divert blood-flow to this layer to help control their temperature.

Their fore-flippers are used primarily for steering, while their hind flippers are bound to the pelvis in such a way that they cannot bring them under their body to walk on them. Because they cannot turn their hind flippers downward, earless seals are very clumsy on land, having to wriggle with their front flippers and abdominal muscles.

Phocids are more streamlined than fur seals and sea lions and can therefore swim more effectively over long distances. They swim by sideways movements of their bodies, using their hind-flippers to their fullest effect (McLaren 1984).

Phocid respiratory and circulatory systems are adapted to allow diving to considerable depths, and they can spend a long time underwater between breaths. Air is forced from the lungs during a dive and into the upper respiratory passages, where gases cannot easily be absorbed into the bloodstream. This helps protect the seal from the bends. The middle ear is also lined with blood sinuses that inflate during diving, helping to maintain a constant pressure (McLaren 1984).

True seals do not communicate by "barking" like otariids. Instead, they communicate by slapping the water and grunting.

Adult phocids vary from 1.17 meters in length and 45 kilograms in weight, in the ringed seal, to 4.9 meters (16 feet) and 2,400 kilograms (5,290 pounds) in the southern elephant seal (McLaren 1984). The male southern elephant seal is the largest seal.

Phocids have a reduced number of teeth compared with land-based members of the Carnivora, although they retain powerful canines. Some species lack molars altogether. The dental formula is:

2-3.1.4.0-2

1-2.1.4.0-2

Feeding and reproduction

Phocids in Argentina

While otariids are known for speed and maneuverability in the water, phocids are known for efficient, economical movement. This allows most phocids to make long foraging trips to exploit prey resources that are far from land, whereas otariids are tied to rich upwelling zones close to their breeding sites. A pregnant female earless seal spends a long period of time foraging at sea, building up her fat reserves and then returns to the breeding site and using her stored energy reserves to provide milk for her pup. The common seal, Phoca vitulina, displays a reproductive strategy similar to those of otariids in which the mother makes short foraging trips between nursing bouts.

Because a phocid mother's feeding grounds are often hundreds of kilometers from the breeding site, she must fast while she is lactating. This combination of fasting with lactation is one of the most unusual and extraordinary behaviors displayed by the Phocidae, because it requires the mother seal to provide large amounts of energy to her pup at a time when she herself is taking in no food (and often, no water) to replenish her stores. Because they must continue to burn fat reserves to supply their own metabolic needs while they are feeding their pups, phocid seals have an extremely thick, fat-rich milk that allows them to provide their pups with a large amount of energy in as small a period of time as possible. This allows the mother seal to maximize the efficiency of her energy transfer to the pup and then quickly return to sea to replenish her reserves. The length of lactation in phocids ranges from 28 days in the northern elephant seal to just 3 to 5 days in the hooded seal. The nursing period is ended by the mother, who departs to the sea and leaves her pup at the breeding site. Pups will continue to nurse if given the opportunity, and "milk stealers" that suckle from unrelated, sleeping females are not uncommon; this often results in the death of the pup whose mother the milk was stolen from, as any single female can only produce enough milk to provision one pup.

The pup's diet is so high-calorie that the pup builds up a large store of fat. Before the pup is ready to forage on its own, the mother abandons it, and it lives on its fat for weeks or months while it develops independence. Seals, like all marine mammals, need time to develop the oxygen stores, swimming muscles, and neural pathways necessary for effective diving and foraging. Seal pups typically eat no food and drink no water during the fast, although some polar species have been observed to eat snow. The post-weaning fast ranges from two weeks in the hooded seal to 9 to 12 weeks in the northern elephant seal. The physiological and behavioral adaptations that allow phocid pups to endure these remarkable fasts, which are among the longest for any mammal, remain an area of active study and research.

Evolution

The earliest fossil phocids date from the mid-Miocene, 15 million years ago in the north Atlantic. Until recently, many researchers believed that phocids evolved separately from otariids and odobenids from otter-like animals, such as Potamotherium, which inhabited European fresh-water lakes. Recent evidence strongly suggests a monophyletic origin for all pinnipeds from a single ancestor, possibly Enaliarctos, most closely related to the bears.

Monk seals and elephant seals are believed to have first entered the Pacific through the open straits between North and South America, which closed only in the Pliocene. The various Antarctic species may have either used the same route, or traveled down the west coast of Africa (Savage and Long 1986).

Classification

In the 1980s, phylogenetic analysis of the phocids lead to a few conclusions about the interrelatedness of the various genera. The four genera Hydrurga,Leptonychotes,Lobodon, and Ommatophoca form a monophyletic group, the tribe Lobodontini. Likewise, the Phocinae subfamily (Erignathus,Cystophora,Halichoerus, and Phoca) is also monophyletic. (More recently, five species have been split off from Phoca, forming three additional genera.) However, the family Monachinae (the lobodonts plus Monachus and Mirounga is probably paraphyletic (Wozencraft 2005).

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